The invention relates to a motor vehicle comprising a low exhaust heat engine for driving the driven wheels, in particular an electric engine possibly combined with a heat engine. In the latter case, the vehicle is commonly qualified as a hybrid vehicle.
As with heat engine motor vehicles, electric or hybrid motor vehicles must integrate a system for conditioning the air temperature of the passenger compartment. These systems for conditioning make it possible to ensure the comfort of passengers as well as additional functions such as the demisting and the defrosting of glass surfaces.
Systems for conditioning the passenger compartment of heat engine vehicles consume a quantity of energy that is incompatible with that available in electric or even hybrid vehicles, in the sense wherein for the latter, the heat engine may be stopped over long durations.
Indeed, in conventional heat engine vehicles, the abundant engine heat losses are at a sufficiently-high temperature to be directly used for heating the passenger compartment, through an inexpensive and very compact system. The relatively massive amount of energy needed for heating (up to 10 KW) are as such satisfied free.
For air conditioning, there are also powerful and compact systems, of which the core is a compressor mechanically driven by the engine, that does not have any disadvantage in use other than a substantial overconsumption of fuel (+3 to 4% on an annual average in temperate regions), but is moreover accepted in light of the advantages in terms of comfort and safety that are procured, as these vehicles do not have any limited autonomy.
Nevertheless, when viewed from a sustainable development standpoint, automobile air conditioning is however criticised, due to this overconsumption and the contribution of the coolant gases implemented to global warming.
Indeed, this extended cooling loop that is distributed in the engine compartment, contains a large quantity of coolant gas, a major contributor to the greenhouse effect and which is difficult to confine during the life of the vehicle.
Controlling the diffusion of this gas into the atmosphere is illusory in that any mechanical intervention concerning the power train requires the cooling loop to be purged, and that the condenser—exchanger wherein this gas circulates, on the front side of the vehicle, is one of the first units to be damaged in the event of a frontal impact.
In contrast with heat engine vehicles, electric or hybrid vehicles have an autonomy in pure electric traction, with heat exhaust that quantitatively is highly insufficient in relation to heating needs, which in addition is at a temperature that is too low to be useable directly.
In air conditioning, on these same vehicles, the drive energy for the compressor has a strong impact on the autonomy in town (in pure electric mode with the hybrid case).
For these new engines, that aim to be environmentally-friendly, non-polluting in town and as independent as possible from non-renewable sources of energy, it is coherent to find solutions that do not discharge anything into the atmosphere, that consume a minimum amount of on-board energy, and that comply with imperatives concerning cost, confined installation and mass, which characterize the automobile.
Adding that it cannot be considered to restrict in any way the level of comfort that modern motorists are used to, or to impose new restrictions that mobilize their time or limit the availability of the vehicle, it appears that no suitable overall solution exists to date, or is even proposed, to satisfy the thermal conditioning needs of the passenger compartment of electric or hybrid vehicles having an autonomy in pure electric traction.
Publications, experimental and even commercial realizations, coming from institutional laboratories, manufacturers or motor vehicle equipment manufacturers have abounded for a few years now, showing the intensity of the search for solutions adapted to these new engines, and more generally to reduction in the impact on the environment of automobile air conditioning.
The solutions that are currently implemented on hybrid vehicles that cannot be recharged via the mains, of which the operating durations with the heat engine stopped remain modest, aim to improve and add to the traditional arrangements of heat engine vehicles, in order to maintain the comfort during the periods when the heat engine is stopped.
For heating, the high thermic inertia of the internal combustion engine allows the comfort to be maintained for a certain period of time. This duration can be usefully prolonged thanks to an extra electric, typically on the air entering into the passenger compartment. When the engine cools down, or when the battery is too discharged, the heat engine is automatically restarted.
With air conditioning, the thermic inertia that can be returned is low and must be quickly relayed. The currently retained solution is a double compressor, delivering its full power with mechanical drive, and power for maintaining, which is more modest, with electric drive. It is naturally more costly and takes up more space than a simple mechanical compressor. Here too, the heat engine is automatically restarted if the comfort demand is no longer ensured (when it is very hot and there is a lot of sunshine) or when the battery is too discharged.
For hybrids of the “stop & start” type, wherein the heat engine is only stopped when the vehicle is stopped, over durations that are generally short, a complement of thermic inertia on the cooling loop, typically in the form of a latent heat thermal reserve can suffice.
Electric or hybrid vehicles that have a substantial amount of electric autonomy, which is currently not widely available, are equipped as follows:                For heating, an electric resistance on the air or on the water, or a boiler of the additional boiler type for “great cold” vehicles.        
The first solution provides services that are clearly lower than those of a thermal vehicle while considerably reducing the pure electric autonomy. The second, that burns fuel and that releases CO2 and other atmospheric pollutants, acceptable on a hybrid despite its cost, is hardly homogenous with the purpose of an electric vehicle.                For air conditioning, a traditional cooling loop, with a small electric compressor.        
Here too, the compromise is often services that are more modest than those of a thermal vehicle, at the price of a substantial reduction in autonomy concerning electric traction, especially in town.
The insufficiency of the solutions available for electric and hybrid vehicles with high electric autonomy lead to consider that the sustainable solution for the future that is suited to this type of vehicle may entail the use of a reversible heat pump.
In its principle, the heat pump—which comprises a cooling loop such as that used in air conditioning, but which can be reversed in the winter—makes it possible, in a single system, to provide for the needs in heating as well as those for air conditioning in the passenger compartment.
Borrowing most of the energy needs from the surroundings, the heat pump has a high degree of efficiency, expressed as the ratio between the energy returned to the passenger compartment and the on-board energy that is consumed. Also referred to as the coefficient of performance or COP, this ratio can currently fall between 2 and 3 for air conditioning, and a bit more for heating. This can provide heating and air conditioning services that are identical to those of a thermal vehicle by devoting a tolerable portion of the energy of the traction battery.
The principle of the vapour-compression heat pump is well known, and has numerous applications, especially in housing.
It is also known that the efficiency of this cycle drops sharply when the difference in temperature between the exhaust side (exchanger with the outside air) and the distribution side (exchanger with the air introduced into the passenger compartment) increases. In automobile air conditioning, this difference is typically about 70° C. (70-80° C. hot side, and 0° C.-10° C. cold side). In heating, the figure reaches 110° C. and goes down under −30° C. cold side.
The use of a heat pump for heating consequently implies considerable oversizing of the compressor, with the normal coolants not very well adapted to such a temperature range. In housing, the use of large exchange surfaces makes it possible to very substantially reduce this amplitude, which is naturally impossible in the confined context of an automobile.
Manufacturers and motor vehicle equipment manufacturers are counting on the future supercritical CO2 cycles, whereon the latter are concentrating the development and investment efforts, in order to resolve this difficulty. Such heat pumps will not become available for several years.
Other types of heat pumps continue to be the subject of research and publications. The constantly-increasing levels of performance with some of them may have them considered as possible alternatives to gas-compression heat pumps: magnetocaloric material heat pump, Stirling cycle heat pump, in particular.